Onderstepoort Jour·nal of Veterinaty Science and A.nimal lndustty, Voltnne 15, Numbers 1 and 2, July and OctobM, 1940.
Printed in the rnion of South Africa by th2 Government Printer, Pretoria.
Studies on the Basic Characteristics of South African Merino W ool.-1. Breaking Strength and Tensile Strength.
By V. BOSMAN, EhEAKOR A. WATERS'fON and C . .M. VAN WYK, Wool Research Section, Onderstepoort.
THE basic characteristics of the raw material hom which textiles are manufactured largely determine the characteristics of the finished doth, and a knowledge of the basic characteristics of the raw material is valuable.
It has already been shown (Bosman, 1937) that the Union of South Africa produces an extremely fine fibred type of Merino wool which is used as raw material in the manufacture of fine fibred
fabrics.
As regards the other characteristics, little experimental data have been available and information regarding South African wool could not he given to the users of the raw product. In addition, the wooL producer has not been able to compare his product with other textile fibres, nor has he known how to effect improvements in hiH raw material.
Other textile organisations, such as those that control the manufacture of synthetic fibres, are in a stronger position in this respect, since by constant research they are able to say precisely what the intrinsic characteristics of their products are. In their ease the continued improvemr.nt in the characteristics of synthetic fibres is dne to intensive researches.
This position was realised by the South African Wool Council and research projects for studying the basic characteristics of South African Merino wool were financed hv i he Council out of Wool LevY
funrls. ' ·
'l'he present contribution (leals with tensile strength. Other characteristics are deseribed elsewhere.
Apart from the actual experimental. resalts presented in these publications, a great deal of work has had to be undertaken to evolve suitable methods for expressing the characteristies in arithmetic and
313
BASIC CHAltACTElUS'l'ICS 01:' MERINO WOOL.
comparable terms. This system is also essential in research work that deals with Merino wool from production aspects and it forms the basis for further work on researches into Merino genetics and nutrition.
REVIEW OF LITERATURE.
Although several publications, dealing with the breaking strength and tensile strength of wool from different breeds of sheep are available, no work dealing directly with the breaking strength and tensile strength of South African Merino wool is known.
The first reliable estimate of the breaking load of wool appean;
to he recorded by McMutrie (1886) who gave average values for Yarious breeds of sheep. Matthews (1904) designed an instrument for determining breaking strength and considered that an average of 10 fibres per sample gave a reliable estimate of the characteristic.
Hill (1908, 1911, 1912) concluded that an average obtained from a 1,000 fibres per sample was not reliable and considered that the breaking strength is more nearly proportional to the diameter of the fibre than to the square of the r1iameter.
:Yiiller and 'l'allman (1915) studied the tensile strength of wool fibres, testing 1,000 fibres per sample. Guldenpfennig (1915) studied the breaking strength of fibres from pure-bred and cross-bred sheep.
Hardy (1918, 1920) concluded that the tensile ~trength of wool, both in the grease and as scoured, decreased with an increase in relative humidity from 40 per cent. to 80 per cent. and showed a tendency to increase thereafter to saturation. Hardy stressed the need for diameter measurements and considered that the smallest diameter should be used in calculating tensile strength. He found an increase in the tensile strength with a decrease in fibre diameter and asserted that the breaking strength of fine wool nried more closely with the first than with the second power of the diameter, while the breaking strength of coarse wool varied with a figure lying somewhere between the first and seconrl powers of the diameter.
Kronacher (1924) found a high correlation between fineness and breaking strength and gave standards of bre:1king strength for the various fineness classes.
Kiihler (1924) found a higher tensile strength for fine than for coarse wools in the case of Karakul sheep, his conclusions being verified by Dimitriades (1926) who used wool hom }ferino yearlings.
Deppe (1926) found a high correlation between finfness and breaking strength. Further investigations were carrieo out bv Tanzer ( 1926) and Anert (1929), the latter using the smallest finene'ss value for the calculation of tensile strength. Krais (1927), whilst reYiewing the testing· methods used at Dresden, gaye instan<'fl'l of the value of tensile strength determinations.
Ogrizek (1926) and Constantinescu and Contescu (1928) found correlations between breaking strength and diameter in a study of the wool of Zigaja sheep.
314
V. llOSMAN, E. A. WATBRST05 A5ll C. :u. VAN WYIL
Barker and Hedges (1927) found a decrease of approximately 0 ·57 per cent. in the breaking strength of yarns for each 1 per cent.
rise in relative humidity. Reimers and Swart (1930) studied the relation between diameter, extensibility and carrying capacity of fibres from a number of closely related Merino rams. Saur (1931) advocated the standardisation of the duration of the time of tests on tensile strength.
The variation of tensile strength and extensibility and their eorrelation with fibre fineness were studied by Karrner (1932).
Doehner (1932) devised an apparatus for determining the tensile
.~trength of a bundle of fibres, and later (1935) applied the instrument
in a study of the monthly and yearly variations of tensile strength in the wool of sheep of the most important breeds occurring in Germany.
Cunliffe (1933) published photographs of fibres under stress whiclt ,;howed clearly the diminution of fibre diameter during stretching.
Schmidhauser (1936) compared the tensile strength, extensibility and fineness of various textile fibres including staple :fihrf'.
An extensive series of experiments on the elastie properties of wool was commenced by Speakman (1924, 1926, 1927, 1928, 1929, 1930 and 1931) using English Cotswold wool. He was able to form a picture of the structure of the fibre and explain its properties.
The work was supplemented by X-ray studies (Ewles and Speakman 1928 and 1930).
ExPERHfENTAL.
The Selection of Sarrnples cmd Methods of A nalys·i.,.
The South African wool clip is produced under varying conditions of climate and pasturage and a representative selection of the clip must take into account the types of ·wool producerl in the rlifferent wool-growing areas.
'!.'he question of how a rep1·esentative selection of samples from the South African Merino clip could be obtainerl was carefully considered. Assuming that there are approximately 50,000 Union farmers who sell wool, the question of obtaining samples from each of these clips would make a study of this nature prohibitive. A reasonable representative selection of samples ('otllrl be obtained, however, by choosing representative samples from distinctive wool growing a1·eas. The range of samples used for the study therefore includes types grown on grass-veld, mixed-veld anrl karroo-veld and were obtained from wool growing areas of the four proYinces of the Pnion.
'fhe laboratory determinations of the series of samples were carried out on a representative selection of fibres taken from a larger lot of wool, whether this was a farmer's clip, a bale or a lesser rruantity. Each case was treated on its own merits and n sample taken according to the size of the lot.
BASIC CHARACTERISTICS O.F )fERl::'\'0 WOOL.
The instrument used for the i.est was that devised by Doehner (1932). It was found to be very suitable for these studies, since it determines the mean breaking strength of a bundle of fibres and for this reason was preferred to other existing instruments.
Usually the original sample was divided into zones, fifteen to thirty, or more, depending on the size of the sample, and single staples were drawn at random from each zone in succession, until a sub-sample of about 100 grams had been made up. From each of the staples forming a sub-sample, a small strand of fibres was separated off and eleansed in ether, the strands being laid side by side after washing. A single fibre was drawn at random from each strand and after being straightened just sufficiently to eliminate the crimping, was mounted on a frame devised by Doehner (1932) and its ends secm·ed with adhesive wax. When a hundred fibres had been mounted, the ends of the bundle were fixed by sealing wax to a strip of paper perforated by a special instrument that is included in the Doehner appartus. The mean value obtained from three such bundles made up of fibres from the original sub-sample, was shown, by special tests, to be of sufficient accuracy for the purpose of the present study. It was shown that the variation among the three bundles of a sample (expressed by a standard deviation of± · 080) was considerably smaller than the variation among the different samples (the standard deviation of the latter being ± · 231). There was thus a real difference in the tensile strengths of different samples.
All determinations were carried out in a room maintained at a constant relative humidity of 70 per cent. at a temperature of 70° Fahrenheit. (According to tests made on a number of samples, values obtained at 70 per cent. relative humidity may be adjusted to correspond to 65 per cent. relative humidity by adding 0 · 45 ( ± · 21) grams to the breaking strength, and 0 ·14( ± 0 · 04) x 10"
gms.
i
em. 2 to the tensile strength). The paper strips containing the fibres were placed in a desice.ator for 24 hours before being exposed to the conditions of the humiditv cham her. The fibres therefore in all eases contained moisbue corresponding with adsorption conditions.The breaking strength was determined at points two millimeters apart on the bundle of fibres, this operation being aided by the perforations on the paper holding the bundl.es. The number of points of breaking was thus dependent on the length of the bundle of fibres and usually ranged from 3 to 5.
The rate of loading of the iru;trument was ~ kilogrammes per minute g-iving an average of 20 grammes per minute for individual fihres. The load at break was read off directly on the calibrated scale.
vVhen the tests were made, the Onderstepoort Wool Research Laboratories had, for certain reasons, its Constant Humidity Chamber set at 70 per cent. Relative Humidity and 70° Fahrenheit. Since then the conditions have been changed to the internationally adopted standard of 65 per cent. Uelative Humidity and 70° Fahrenheit.
After the fibres, forming one bundle, had been broken, all the fragments were collected and mounted on a slide in Euparal. From eaeh slide two hundred fibre thickness measurements were made at random on a Zeiss Lanameter and the mean cross-sectional area
816
Y. llOSJ\1.\K, E. A. \LI.TEHSTOK AX]) C. ::'.1. VAl' \\"Yli:.
calculated from the mean square of the two hunclred measurements, it being assumed for comparative purposes that the :fibres were eireular in cross-section. From the data on the breaking strength, the number of fibres broken, and the cross-seetiona 1 area. the tensile strength was calculated.
As regards the power of the diameter with whieh the breaking
~trength varies, Hardy (1920) asserted that the breaking strength of :fine wool varied more closely with the first than \l"ith the second power of the diameter, while the breaking strength of eoarse wool Yaried with a figure lying somewhere between the fin;t and the second powers of the diameter. 'This assertion ·was tested on 114 marino samples by ealculating the regression coefficient of the logarithm of the breaking strength on the logarithm of the diameter. The result was 1· 895 (
± ·
0183) which is near enough to 2 to justify the assumption adopted for the present work that the breaking· strt:>ngth Yaries as the square of the fibre diameter.RESULTS.
A. Yilne.s w·ith·in the same Sta[llc.
A portion of the work deals with the breaking strength and teu,.,ile strength of fibres \Yithin the same staple of }[fn·ino wool.
Twenty different Merino staples were selected and the coarse and fine fibres were separated from each other. Each lot of fibres
~eparated was measured for fibre fineness and for straight fibre length. 1'he high degree of correlation between the fibre fineness and the length of the fibres in the same staple shown h~· Duerden ctnd Bosman (1931), viz. values from
+
·91 to+
·99, was also evident in this series 'vhere the c-orrelation was founc1 to be+
·7944. The results of the me::1su!·ement of fibre fineness, hreaking strength and tensile strength are summarised in Table 1.'l'he fibre fineness of the fine and coarse fibres of each stavle
1::; gnen in column 3. The mean breaking strengths per fibre art>
recorded in column 4 \Vhere the degree to which fine and coarse fibres in the same staple differ in breaking strength is demonstratecl.
The average l?reakiug streng-th o E the coarse fibres iR G ·uG ( 1:: · 504) grammes and that of the fine fibres is 4· 88 ( ± >WO) grammes. On an average the coarse fibres are 52 per cent. strong-er per fibre than the fine fibres. When the same load i,., plnced on the different fibres in the staple, the finer ones \\·ill break ,.;ooner and the c·oarser ones will sbncl a load of at least hnlf "" lnrge ag·ain us do the finer filwe~.
The practical application of these facts, whether during the processes of manufacture of wool fabrics, or during wear in wool garments, needs further investigation, although several authors bave already 'contributed to the subject (Cowden, 1927; William;;, 1932, et olia). Williams in discus:-;ing the strength of textile fabric,;, asserts that " the problems of strength were perhaps not so important in lhe past when fabrics were usually heavy and strong, hut now that lighter nnd necessarily IYeaker fahricR nn• in clem;Hnl
BASIC CHARACTERISTICS OF MERINO WOOL.
and synthetic textile fibres of inferior strength to the natural fibres are extensively used, the strength problem is certainly of great.
importance."
'fABLE 1.
The Results of the Measurement of Fib1-e Fineness, Break·ing Strength and Tensile St1·ength of Fibres within the same StazJle
of Merino Wool.
Sample.
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Type of Fibre Selected.
Fine. . . . . . . . . . . . . . . . Coarse .. ...
Fine ..... ...
Coarse .... . . . . .
.
. . . . Fine .......Coarse ............ Fine ........
Coarse ........
Fine ... ··· ..
Coarse ...
Fine .......... Coarse .. Fine ...
Coarse ...
Fine ..........
Coarse .... . . . . . . . . . . Fine ... ...
Coarse .........
Fine .... ... Coarse ... ... ..
Fine ............
Coarse ............ Fine ........ Coarse. ··· ...
Fine ..... Coarse ...
Fine ..... Coarse .. ...
Fine ... ... ··· ...
Coarse. . . . . . . . . . . . Fine ... ...
Coarse . .... .... ... . . Fine ... ...
Coarse .......
Fine ..... Coarse ..........
Fine ... ...
Coarse ...........
Fine .......
Coarse ....
Mean Fineness.
(p.).
17·97 29·02 18·36 22·24 15·25 20·96 17·54 20·71 21·75 26·09 19·37 23·58 15·80 23·45 20·00 25·67 16·35 19·87 19·17 25·60 16·02 19·08 19·00 20·64 17·23 20·57 17·18 21·97 24·81 34·14 23·36 27·50 19·37 27·62 25·03 31·80 18·88 21·33 22·29 25·79
Mean Breaking Strength per Fibre (gms.).
3·40 6·84 4·37 6·26 2·80 4·84 3·97 5·03 6·06 8·60 3·87 5·73 2·95 6·74 5·25 8·22 2·76 5·09 4· 71 7·89 3·33 4·53 3·58 3·66 3·69 4·27 3·45 5·61 5·90 12·08 6·51 8·30 7·54 11·45 4·49 6·91 3·41 4·68 5·46 6·50
Mean Tensile Strength (gms.
per sq. em.
± 10.
1·32 0·99 1·62 1·59 1·44 1·10 1·56 1·47 1·68 1·05 1·29 1·28 1·48 1·53 1·64 1·51 1·28 1·61 1·60 1·48 1·60 1·54 1·24 1·07 1·55 1·25 1·45 1·44 1·20 1·27 1·40 0·97 2·49 1·84 0·86 0·85 1·19 1·27 1·38 1·21
The point, whether large differences in fineness of the fibres in the same staple tend to give large differences in the breaking strength, was investigated. It was found that there was a coefficient of correlation of 0 · 853 between differences of these two characteri~
tics, which finding is significant from a breeding aspect. It haR 318
- - - - -- -- - - -
Y. IJOSMAK, E. A. \L\TEHSTOK AND C. M. VAN WYK.
been shown by Duerden and Bosman (1931) that " a wool very variable in length will also be very variable in thickness, or a wool uniform in length will also, be uniform in thickness. In striving for the uniformity of one att1;ibute, the breeder will tend to attain uniformity of the other,;. One must then also conclude that the breeder, in striving for uniformity of fibre fineness, will. tend to attain uniformity in fibre length and also in the breaking strength o£ the fibres.
As regards the tensile strength, given in the last eolumn of 'l'able 1, the aYerage (expressed as grammes per square em. x 10') was 1· 32 (
± ·
058) for the coarse fibres and 1· 46 (± ·
070) for thefine fibres. The difference (0 ·14 ± 0 · 054) is significant at the 5 per
(~ent. probability level. The fine fibres within the same staple th11R
tend to have a higher tensile strength than the coarser ones. The coefficients of correlation of the fibre characteristicR within the same staple of Merino wool llre rmmmarised in Table 2.
TABLE 2.
Correlat?'on Coefficients.
·Fibre Diameter.
I
Breaking Strength. Tensile Strength.Fibre Diameter ...
I
-+
· 9508 (significant - ·4822 (significant at l per cent.) at 5 per cent.) Breaking Strength ..+
· 9508 (significant-
- ·4338 (significanat l per cent.) at 5 per cent.)
Tensile Strength .... - · 4822 (significant - · 4338 (significiLnt - at 5 per cent.) itt 5 per cent.)
There is a highly significant correlation between fibre diameter and breaking strength (-9508) and a significant negative correlation between fibre diameter and tensile strength (-· 4822) and also between breaking strength and tensile strength (- ·4338). The f'on·elation between fibre diameter and tensile strength shows that the fine fibres in the staple hase a higher tensile strength than the eoarse fibres and suggests strudural differences between the fine and coarse fibres of the staple, a point that is now being further studied frorn such aspects as breeding and nutrition.
In his work on " Fleece Density and the Histology of the )Lerino Skin " Carter (1939) has shown the existence of " primary '' and " secondary" wool-producino· foll.icles and it is probable that the coarse and fine fibres referred to aboYe are produced respediYely from the " primary " and " secondary" follicle and constitute two distinct types in the same staple.
B. A.nalysis of Different Sarnples.
A representative selection of 134 samples of South African
~1erino wool was analysed for breaking strength and tensile strength. The results of the t "·o characteristics, arranged as frequency tables, are summarised respectiYel.y in Tables
a
and 4.BASIC CHARACTERISTICS OF MERINO WOOL.
'hBLB
a.
Thie A•t,e·raoe Breaking Strengths per Fibre of a select·ion of South African Merino 1V ool Samples.
Group lnteMJal (expressed as
grammes).
1 to 2 ... . 2 to 3
' )
tJ to 4
4 to 5 ... ... 5 to 6 ... ...
6 to T ... ... 7 to 8 8 to 9 9 to 10 10 to 11 ... ...
. ..
... ...
... ...
... ... . ..
... ...
...
. ..
. .
. .. . .
Freq1tency.
2 3 8
41
:n
14
]
0 1
Mean: 5·50* grammes (at 70 per cent. Relative Humidity and 70° Fahrenheit).
Standard deviation: 1· 357 grammes. Coefficient of variability : 24 · 67 per een t.
The average breaking strength p~r fibre of the samples range;; from 1 gratnme to ]] grammes "itl1 a mean of 5·50 grammes.
'l'ABLE 4.
The Tenstle Stren.gth of a selection of Sottth Alricwn J!f erino vVoo/
Sa71~<p7 eg.
GrouzJ Interval Freqtt('IU'.'f·
(expressed as grammes
per square cm .. x 106) .
0· 6 to 0 ·7 . . . 1
.0·7to0·8 1
0·8 to 0·9 2
0·9 to 1·0 3
1 · 0 to 1 · 1 . . . 14
1·1 to 1·2 24
1·2 to 1·3 41
1·3 to 1·4 27
1 · 4 1to 1 · 5 HI
1·5 to 1·6 2
Mean: 1·243tx10" gramme,; per sq. em.
Standard deviation: ± 0 ·1552 x 106 grammes per sq. rm.
Coefficient of varia hili ty : 12 · 49 per een t.
*or 5·95 grammes at 65% Relative Humidity and 70° F.
tor 1·383xl0' grammes per square centimE'tre nt 6.'5% HE'lntive Humidity and 70° F.
320
The tensile strength of South African Merino wool. ::;am pie,;
varies from 0 · 6 to 1· 6 ( x 10") grammes per square em. with an average of 1·243 ( x 10") grammes per square em. The latter figure ean also be expressed as 8 tons per square inch 0r 12·4 kil0grammes per square millimetre .
. The values for tensile strength, given in this paper, represent averages of large numbers of fibres (tested in bundles) and have a more flirect bearing on general practice than those determinations that take into account only single fibres. It has been shown by Kiisebauch (1937), who also discusses the advantage of bundle test,;, that the average strength found by single fibre tests is 1 · 0799 times the val.ne obtained by tests of bundles of 100 fibres.
CoRRELATIOl\ s.
~\. study of eonelations in the mnge of South Afri<:au sample,.;
shows that the coefficient of correlat-ion between fibre fineuess and breaking strength is 0 · 896(
± ·
0186) indicating a highly significant relationship behYeen the two characteristir:s.The value for this con&tant between fibre fineness and tensile strength is - ·1780(
± ·
0911) indicating an insignificant correlation. lt eannol;, therefore, he said that in a random selection of Merino wool such as was used for the present study, the finer type(;have it higher or lower tensile strength than the coarser wools. 'fhis eonclusion does not apply to fibres within the same staple (shown above) and it is l.ikely that a random selection of wools from different sources would have undernourished as well as well-grown types, a faetor which might reduce the value of a correlation coefficient.
Further work on this aspect is now in progress.
The regression coeffl cien t of the breaking strength on the fibre finess is 0 · 445 ( ± · 0208) indicating that on an average every increase of lfl- in fibre fineness is associated with an increase of approximately 0 · 445 grammes in the breaking strength. This value is valid over a limited range only, since the relation between breaking st.rettgth and fibre (lianH~ter is not linear.
SuMMARY AND CoNcr.us.roNs.
A series of South .African Merino wool samples, representing·
wool.s from different parts of the Union, was analysed for breaking·
;;trength at1d tensile drength.
'l'he method IJt determination, using Do·:hner's instrument:, consisted of bundle tests, this giving average values for larger samples and lots.
A portion o£ the dnalysis is devoted t0 l he hreabng strengt It and tensile strength of fibres within the same staple. It is shown that the average breaking strength of the coarse fibres within the staple is G · 66 (
± ·
504) grammes. That of the fine fibres is 4 · 38 (± ·300) grammes, so that the coarse fibres are 52 per cent. stronger per fibre tha11 the fine fibres. The practical significan<'e of this is discussedBASIC CHARACTERISTICS OF MERINO WOOL.
Breeding aspects are discussed and it is shown that the Merino breeder, in striving for uniformity of fibre fineness, will tend to attain uniformity in fibre length and also in the breaking strength of the fibres.
The average tensile strength of the coarse fibres within the stapl.e is 1 32(±0·058) and that of the fine fibres is 1·46 (± ·070), (expressed as grammes per square em. x 10"). When fibres within the same staple are considered, there is a significant correlation of
· 9508 ( ± · 7938) between fibre diameter and breaking strength and a significant negative correlation of - ·4822 ( ± ·0456) between fibre ciiamet.er and tensile strength.
'l'he average breaking strength (per fibre) of representative 8outh African Merino wool samples ranges from 1 to 11 grammes with a mean of 5 ·50 grammes.
The tensile strength of South African .Merino wool varies from 0 · G to 1· 6 ( x 10") grammes per square centimetre with au average oi 1· 243 ( x 10") grammes per square centimetre of fibre. The latter figure can also be expressed as 8 tons per square inch or 12 · 4 kilo- grammes per square millimeter of fibre.
When different samples are considered correlation (r = 0 · 896 ± 0 · 0186) between fibre strength, but an insignificant correlation between fibre fineness and tensile strength.
there is a significant fineness and breaking (r= - 0·1780±0·0911)
The regression f'oe:fficient of the breaking l.oad on fibre fineness is 0 · 445 ( ± · 0208) indicating that, on an average, every increase of 1t-t in fibre fineness is asst~ciated with an increase of 0 · 445 grammes in the hrea king strength.
AcKNOW J.EDGE.llENT.
The author~; are indebted to the South African Wool Council for assistance in carrying out the work. This forms a part of a projecL on " 'l'he Basic Characteristics of South African .Merino Wool., which i;;; financed by the Wool Council out of ·wool Levy :Funds.
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